High pressure molding of composite parts

ABSTRACT

Composite pre-forms are molded at high pressure to form composite parts that can be used in place of metal-based high performance parts, such as the outlet guide vanes found in turbofan jet engines. The composite pre-forms include two different fiber orientations that are co-molded in a resin matrix at high pressures to provide composite outlet guide vanes and other high performance parts. Chambers within the composite part are optionally formed during molding of the pre-form at high pressures.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the molding of pre-formedfiber-reinforced resin components into composite parts that havemultiple fiber orientations and/or one or more internal chambers. Moreparticularly, the present invention is directed to molding of suchpre-forms under high pressure.

2. Description of Related Art

Composite materials typically include fibers and a resin matrix as thetwo principal components. Composite materials typically have a ratherhigh strength to weight ratio. As a result, composite materials arebeing used in demanding environments, such as in the field of aerospacewhere the high strength and relatively light weight of composite partsare of particular importance.

High strength metals and metal alloys have been used in the past to formaircraft parts and structures that are subjected to high stress loads.An advantage of using metal is that, in addition to being extremelystrong, metal can be precisely machined so that the geometry of matingparts can be closely matched and tight dimensional tolerances can bemet. Holes can also be precisely machined into metal structures toaccept high tolerance fasteners and positioning pins.

The turbofan engines that are commonly used to propel aircraft include anumber of parts that are subjected to extreme stress and which also mustmeet precise dimensional tolerances. One example of such a part is theoutlet guide vane (OGV). These vanes are airfoil-shaped parts that arelocated in the by-pass area of a turbofan engine. The outlet guide vanesredirect radial airflow from the forward fan assembly into axial airflowthat goes to the turbofan compressor. Each engine can contain as many as100 or more outlet guide vanes. In order to function properly in thisdemanding environment, outlet guide vanes have been traditionally madefrom solid metal or metal alloy. Some outlet guide vanes have been madeby surrounding a lighter core material with high-strength metal. Morerecently, outlet guide vanes have included a metal frame on whichcomposite skins have been mounted. For example, see published U.S.Patent Application No. 2010/0209235 A1. These metal-based outlet guidevanes all tend to be relatively heavy. Accordingly, it would bedesirable to provide a lighter-weight outlet guide vane that is madecompletely from composite material and which has the same structuralproperties and machining characteristics as the metal-based outlet guidevanes.

Making an outlet guide vane from composite materials presents a numberof significant challenges. In order to ensure that the finished part isfree of voids or other defects, it is generally necessary to use moldingtechniques in which pressure is applied to the composite material duringthe molding process. However, it is very difficult to produce moldedcomposite parts that have tolerances which are as tight as thetolerances that can be obtained by machining a molded metal part. Italso is difficult to machine composite parts without creating structuraldamage. The fibers tend to be disrupted and delamination can occur whichgreatly reduces the strength of the part. Composite parts that containunidirectional (UD) fibers are commonly used in many structural partsdue to the unique structural and strength properties provided by a UDfiber orientation. The high directional strength provided by UD fibersmakes this orientation particularly attractive for use in making outletguide vanes. However, composite parts that contain UD fibers areparticularly difficult to machine without adversely affecting thestrength of the part.

Another significant challenge is that the central airfoil of the outletguide vane and the mounting flanges or platforms that connect theairfoil to the engine have unique and different design requirements. Forexample, the airfoil needs to be structurally strong to withstand highairflow loads. The flanges must be strong enough to hold the airfoil inplace and they must also meet strict dimensional tolerances to providesecure and precise mounting to the engine.

A composite material has been developed that can be machined accuratelyto strict dimensional tolerances. This composite material is composed ofrandomly oriented segments of unidirectional tape that have beenimpregnated with thermosetting resin. This type of quasi-isotropic fibermaterial has been used to make high pressure molds and a variety ofaerospace components. The material is available from Hexcel Corporation(Dublin, Calif.) under the trade name HexMC®. Examples of the types ofparts that have been made using HexMC® are described in U.S. Pat. Nos.7,510,390; 7,960,674 and U.S. patent application Ser. No. 12/856,210.

There is a continuing need to develop new processes and procedures formaking composite parts that can be used in place of outlet guide vanesand other high performance parts that have traditionally been made frommetal.

SUMMARY OF THE INVENTION

In accordance with the present invention, outlet guide vanes areprovided that are made entirely from composite materials. The outletguide vanes include a molded body that is composed of fiberreinforcement and a cured resin wherein the molded body includes acentral airfoil portion and two solid mounting flanges or platforms thatform the end portions of the molded body. As a feature of the invention,the fibers located within the central airfoil portion are unidirectionaland the fibers located within the end portion are randomly orientedsegments of unidirectional fiber tape. It was discovered that these twodifferent fiber orientations could be combined and co-molded in a resinmatrix at high pressures to provide composite outlet guide vanes thatmeet the structural strength and dimensional requirements for both thecentral airfoil portion and mounting flanges. This discovery thatunidirectional fibers and randomly oriented segments of unidirectionaltape can be co-molded to provide composite parts with varied localizedproperties is not limited to outlet guide vanes. Instead, the discoveryis applicable to a wide-range of parts where one portion of the partrequires the structural strength and stiffness provided byunidirectional fibers and another portion of the part must meet strictdimensional tolerances that typically require machining of the part.

As another feature of the invention, composite outlet guide vanes areprovided where the weight of the vane is reduced by forming a hollowchamber inside of the central airfoil portion. The hollow outlet guidevane is made by molding a pre-form that has a shape which closelymatches the shape of the outlet guide vane. The pre-form or moldablebody is composed of fiber reinforcement and uncured thermosetting resin.The uncured thermosetting resin has a curing temperature at which theresin is converted from a moldable resin to a solid resin having a glasstransition temperature (T_(g)) that is well-above the curingtemperature. A solid mandrel is located within the pre-form so as toform a chamber inside of the central airfoil portion. The solid mandrelis composed of a material that melts to form a liquid at a meltingtemperature that is above the curing temperature of the thermosettingresin, but below the glass transition temperature of the cured solidresin. It was discovered that solid mandrels that melt above the curingtemperature are required for high pressure molding of combinedcomposites to prevent distortion of the mandrel material and seepage ofliquid mandrel material at the high pressures required to providesuitable composite parts. The mandrel material must also melt below theglass transition temperature of the cured part. Otherwise, the mandrelmaterial cannot be melted and removed from the airfoil chamber withoutdamaging the resin matrix of the part.

The discovery that a fusible mandrel can be used in high pressuremolding of composite materials to form hollow chambers is also notlimited to outlet guide vanes. Instead, the discovery is applicable to awide range of parts where one or more internal chambers are desired inorder to limit weight or to meet some other design requirement.

The present invention covers composite outlet guide vanes as well asother composite parts that employ the above described features of theinvention. The invention also covers the pre-forms or moldable bodiesthat are molded to form the final composite parts. In addition, theinvention covers the methods for molding the moldable bodies at highpressure to form the final parts.

The above described and many other features and attendant advantages ofthe present invention will become better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified view of a jet engine that shows where the outletguide vanes are located within the jet engine.

FIG. 2 is a perspective view of an exemplary outlet guide vane thatincludes a hollow central airfoil portion made from resin impregnatedunidirectional fibers and two solid flange end portions that are madefrom resin impregnated segments of unidirectional tape that are randomlyoriented.

FIG. 3 is a sectional view of a hollow central airfoil portion inaccordance with the present invention where there is a single chamber.

FIG. 4 is a sectional view of a hollow central airfoil portion inaccordance with the present invention where there are two chambers thatare separated by a central spar.

FIG. 5 is a view of a outlet guide vane pre-form as it is being placebetween two mold halves for molding to form an outlet guide vane.

FIG. 6 is a sectional view of a solid outlet guide vane in accordancewith the present invention where a protective metal layer has beenadhered to the leading edge of the central airfoil portion duringmolding of the outlet guide vane.

FIG. 7 is a fusible mandrel for use in forming the chamber within theoutlet guide vane airfoil. The mandrel is made from a tin-zinc eutecticalloy.

FIG. 8 is an outlet guide vane pre-form that has been formed on thefusible mandrel shown in FIG. 7. The pre-form is made up of acombination of unidirectional fibers in the central airfoil portion andrandomly oriented segments of unidirectional fiber tape and athermosetting resin that cures at 196° C.

DETAILED DESCRIPTION OF THE INVENTION

The high pressure molding process of the present invention may be usedto fabricate a wide variety of composite parts where it is desirable tocombine unidirectional fibers and randomly oriented segments ofunidirectional tape in a single part to meet various structuralrequirements while at the same time also meeting various dimensionaltolerances. The process may also be used to locate one or more chamberswithin the composite part. The invention is directed to high-pressuremolding of pre-forms that are composed of uncured thermosetting resinand fibrous reinforcement. High pressure molding utilizes pressures inthe mold of 500 psi to 2000 psi.

Examples of composite parts that can be made using the molding processof the present invention include outlet guide vanes for jet engines,thrust reverser cascades, various engine airfoils, access doors,brackets, flanges and stiffeners for aerospace structures. The followingdetailed description is directed to processes and procedures for moldingoutlet guide vanes from resin impregnated pre-forms. However, it will beunderstood that the processes and procedures described herein may beused to make other similar solid and chambered composite parts.

A simplified turbofan jet engine is shown generally at 10 in FIG. 1. Theturbofan jet engine 10 includes a fan assembly 12 that is located aheadof the turbofan jet engine core 14. Airflow that is discharged from thefan assembly 12 is channeled to a compressor in the core 14 where it isfurther compressed. The compressed air is channeled into a combustorwhere it is mixed with fuel and ignited to form hot combustion gases.The combustion gases are channeled to a turbine that extracts energyfrom the gases to power the compressor and produce power for propellingthe aircraft. Outlet guide vanes 16 are placed in the turbofan jetengine 10 to redirect the radial air discharged from the fan assembly 12into axial air flow in the bypass area of the turbofan jet engine 10.The outlet guide vanes 16 include a central airfoil portion that islocated between mounting flanges. The flanges are used to securelyconnect the outlet guide vanes 16 to the turbofan jet engine.

An exemplary all composite outlet guide vane is shown at 20 in FIG. 2.Outlet guide vane 20 includes a hollow central airfoil portion 22 andtwo solid flanges 24 and 26 for attaching the airfoil 22 to the turbineengine. As shown in FIG. 3, the airfoil portion 22 includes a chamber 28that is formed during the molding process. An alternate exemplary allcomposite outlet guide vane 30 is shown in FIG. 4. The outlet guide vane30 includes two chambers 32 and 34 which are separated by a centrallylocated spar 36 that runs the length of the airfoil portion. A thirdexemplary outlet guide vane is shown at 60 in FIG. 6. The outlet guidevane 60 has a solid central airfoil 62 and solid flanges (not shown).The outlet guide vane 60 also includes a protective metal shield 64 thatis bonded to the leading edge of the vane.

In accordance with the present invention, the outlet guide vanes aremade by high-pressure molding of a composite pre-form or moldable bodythat is a composite material composed of uncured resin and fiberreinforcement. In FIG. 8, an exemplary pre-form 40 is shown that can bemolded to form the outlet guide vane. The pre-form 40 is shaped toclosely resemble the final outlet guide vane. An example of an outletguide vane is 20 cm long and 5 cm wide with thicknesses of the airfoilportion varying from 0.1 to 0.5 cm. For such an outlet guide vane, thepre-form should be undersized from 0.2 to 1.5 cm in all dimensions,except for thickness. The pre-form 40 has a central airfoil portion 42,which may or may not include a fusible metal mandrel depending onwhether a hollow airfoil is desired. The pre-form also includes solidend portions or flanges 44 and 46.

The uncured resin used in the pre-form 40 may be composed of any of thethermosetting or thermoplastic resins that are typically used forstructural applications. Preferably, the amount of uncured resin matrixwill be between 25 to 45 weight percent of the overall weight of thepre-form. The uncured resin matrix may be any of the epoxy resins,bismaleimide resins, polyimide resins, polyester resins, vinylesterresins, cyanate ester resins, phenolic resins or thermoplastic resinsthat are used in structural composite materials. Exemplary thermoplasticresins include polyphenylene sulfide (PPS), polysulfone (PS),polyetheretherketone (PEEK), polyetherketoneketone (PEKK),polyethersulfone (PES), polyetherimide (PEI), polyamide-imide (PAI).Epoxy resins that are toughened with a thermoplastic, such as PES, PEIand/or PAI, are preferred resin matrices. Resins that are typicallypresent in UD tape of the type used in the aerospace industry arepreferred. Exemplary thermoplastic toughened resins that are suitablefor use as the resin matrix are described in U.S. Pat. Nos. 7,754,322and 7,968,179 and U.S. patent application Ser. No. 12/764,636.

Thermoplastic resins cannot be used when chambers are to be formed inthe part using a fusible mandrel in accordance with the presentinvention. Thermoplastic resins do not cure to form a solid resin thathas a glass transition temperature which is sufficiently above thecuring temperature to allow melting and removal of the fusible mandrelwithout damaging the thermoplastic part. Accordingly, thermosettingresins must be used when forming one or more chambers in the compositepart. Thermosetting resins that are toughened with a thermoplastic maybe used to form chambered parts. Epoxy resins that are toughened withthermoplastic are preferred.

The uncured resin and fiber reinforcement may be added to the pre-formseparately. However, it is preferred that the fibers be pre-impregnatedwith resin prior to being used to make the pre-form. Suchpre-impregnated fibers are commonly referred to as “prepreg”. Thepre-form is prepared by forming pieces of prepreg into the desiredpre-form shape. When a fusible mandrel is used, the various pieces ofprepreg are placed uniformly around the mandrel.

Randomly oriented segments of unidirectional tape that are impregnatedwith resin are commonly referred to as quasi-isotropic chopped prepreg.Quasi-isotropic chopped prepreg is a form of random discontinuous fibercomposite (DFC) that is available commercially from Hexcel Corporation(Dublin, Calif.) under the trade name HexMC®. As previously mentioned,HexMC® has been used for a variety of purposes including aerospacearticles and high-strength molds.

Quasi-isotropic (Q-I) prepreg is composed of segments or “chips” ofunidirectional fiber tape and a resin matrix. Q-I prepreg is typicallysupplied as a mat made up of randomly oriented chips of choppedunidirectional tape prepreg. The size of the chips may be varied as wellas the type of fibers depending upon the size and shape of the pre-formas well as how precisely the pre-form must be machined to meetdimensional tolerances, if any. It is preferred that the chips be ⅓ inchwide, 2 inches long and 0.006 inch thick. The chips includeunidirectional fibers that can be carbon, glass, aramid, polyethylene orany of the fibers types that are commonly used in the aerospaceindustry. Carbon fibers are preferred. The chips are randomly orientedin the mat and they lay relatively flat. This provides the mat with itstransverse isotropic properties.

The UD tape prepreg that is chopped to form the chips or segmentsincludes a resin matrix that can be any of the resins mentionedpreviously that are commonly used in aerospace prepregs. Thermosettingepoxy resins that are toughened with thermoplastics are preferredbecause they tend to be more resistant to fracturing or delamination ifmachining of the final composite part is required. The resin content ofthe chips may also be varied between 25 and 45 weight percent of thetotal prepreg weight. Chips with resin contents of between 35 and 40weight percent are preferred. No additional resin is typically added tothe prepreg chips when forming the quasi-isotropic chopped prepreg. Theresin present in the initial UD tape prepreg is sufficient to bond thechips together to form the mat.

The quasi-isotropic (Q-I) chopped prepreg can be made by purchasing ormaking unidirectional prepreg tape or tow of desired width. The tape ortow is then chopped into chips of desired length and the chips are laidrandomly in layers to form the solid portions of the pre-form or laidrandomly in uniform layers around the mandrel in those portions of thepre-form that are chambered. The randomly placed UD prepreg chips arepressed together to form the pre-form. The pre-form may be composedentirely of Q-I prepreg chips when a mandrel is present in the pre-form.Otherwise, the randomly oriented UD prepreg chips are used to form onlya portion of the pre-form with the other portion being composed of UDprepreg and/or other fiber orientations. When pressed together, theindividual randomly oriented UD prepreg chips inherently bond togetherdue to the presence of the prepreg resin. The preferred method, however,is to purchase HexMC® or equivalent commercially availablequasi-isotropic chopped prepregs, which are supplied as sheets ofmaterial that are then used to form the solid portions of the pre-formand/or the chambered portions of the pre-form.

An exemplary preferred quasi-isotropic chopped prepreg material isHexMC® 8552/AS4. This quasi-isotropic chopped prepreg material issupplied as a continuous roll of a mat that is 46 cm wide and 0.20 cmthick. HexPly® 8552/AS4 unidirectional fiber prepreg is used to make thechips that are randomly oriented in the quasi-isotropic mat. HexPly®8552/AS4 prepreg is a carbon fiber/epoxy unidirectional tape that is0.016 cm thick and has a fiber areal weight of about 145 grams/squaremeter. The resin content of the tape is 38 weight percent with the resin(8552) being a thermoplastic-toughened epoxy. The tape is slit toprovide 0.85 cm strips and chopped to provide chips that are 5 cm long.The chip density is about 1.52 gram/cubic centimeter. Other exemplaryquasi-isotropic chopped prepreg can be made using other HexPly®unidirectional prepreg tape, such as EMC 116/AS4 (epoxy/carbon fiber),8552/1M7 (thermoplastic-toughened epoxy/carbon fiber), 3501-6/T650(epoxy/carbon fiber) and M21/IM7 (thermoplastic-toughened epoxy/carbonfiber). HexMC® 8552/AS4 and M21/IM7 are preferred quasi-isotropicchopped prepregs for use alone, or in combination with other fiberorientations, to form the pre-forms in accordance with the presentinvention.

Woven fiber fabric and other fiber orientations may be used incombination with the randomly oriented UD prepreg chips to make thepre-form. However, it is preferred that unidirectional fibers are used.The UD fibers may contain from a few hundred filaments to 12,000 or morefilaments. UD fibers are typically supplied as a tape made up ofcontinuous fiber in a unidirectional orientation. UD tape is thepreferred type of prepreg that is used to form the fibrous structure.Unidirectional tape is available from commercial sources or it may befabricated using known prepreg formation processes. The dimensions ofthe UD tape may be varied widely depending upon the particular compositepart being made. For example, the width of the UD tape (the dimensionperpendicular to the UD fibers) may range from 0.5 inch to a foot ormore. The tape will typically be from 0.004 to 0.012 inch (0.01 to 0.03cm) thick and the length of the UD tape (the dimension parallel to theUD fibers) may vary from 0.5 inch (1.3 cm) up to a few feet (one meter)or more depending upon the size and shape of the pre-form and theparticular orientation of each piece of UD tape within the pre-form.

A preferred exemplary commercially available unidirectional prepreg isHexPly® 8552, which is available from Hexcel Corporation (Dublin,Calif.). HexPly®8552 is available in a variety of unidirectional tapeconfigurations that contain an amine cured toughened epoxy resin matrixin amounts ranging from 34 to 38 weight percent and carbon or glass UDfibers having from 3,000 to 12,000 filaments. The fibers typicallyaccount for 60 volume percent of the UD tape. The preferred UD fibersare carbon fibers.

When making a solid outlet guide vane of the type shown at 60 in FIG. 6,UD tape prepreg is laid longitudinally to form a solid airfoil portionwhere the UD fibers extend between the mounting flanges on either end ofthe airfoil. The pre-form flanges are formed using randomly oriented UDprepreg chips. The two fiber orientations should be overlapped toprovide a strong junction. For example, the randomly oriented UD prepregchips can be placed so that they extend into the airfoil portion andoverlap the UD tape prepreg at the ends of the airfoil portion. Thedegree of overlap should be sufficient to eliminate any voids orresin-rich areas that could lead to cracking at the junction between theUD fibers in the airfoil and the randomly oriented UD prepreg chips thatmake up the flanges. It is more preferred that the UD fibers areextended into the flange portions to provide the needed overlap. Ineither case, it is preferred that the various layers of UD prepreg andUD prepreg chips are interleaved in alternating layers to providemaximum strength at the joint where the two different fiber orientationsmeet and are overlapped.

As shown in FIG. 5, the solid pre-form is placed between two mold halves52 and 54 and heated to the curing temperature of the resin and moldedat high pressure to form the outlet guide vane 60. Typical high-pressurecuring temperatures for epoxy resins range from 170° C. to 225° C.Preferred curing temperatures range from 190° C. to 205° C. Internalpressures within the mold are preferably above 500 psi and below 2000psi at the cure temperatures. Once the pre-form has been completelycured (typically 10 minutes to 1 hour at curing temperature), the partis removed from the mold and cooled to form the final part. If required,the flanges or platforms may be machined to form the final flange shapeand provide any precise dimensions that are required.

Preferably, the pre-form is “staged” prior to being placed in the moldin order to increase the viscosity of the resin to help maintain thepre-form shape and keep the mandrel in place during subsequent highpressure molding. Staging involves heating the pre-form at ambientpressure to a temperature of 165° C. to 180° C. for just enough time tosubstantially increase the viscosity of the resin. Staging times on theorder of 5 to 15 minutes at the staging temperature are preferred. Thestaged pre-form is preferably cooled to room temperature prior to beingplaced in the mold for final curing. In addition, the viscosity of theresin in the pre-form tends to drop as the pre-form is heated to curetemperature and then rapidly increases as the resin cures. It ispreferred that the mold not be pressurized until after the resin hasreached the minimum viscosity. In practice, the staged pre-form isplaced in the mold, which has already been heated to the curingtemperature. Pressurization of the mold is delayed from a few seconds toa minute or more in order to allow the resin time to move through theminimum viscosity phase. This delay in pressurization is particularlypreferred for pre-forms that include a fusible mandrel for forminginternal chambers in order to limit movement of the mandrel whenpressure is applied.

The method for forming outlet guide vanes that have hollow internalchambers is basically the same as the method described above for makingsolid outlet guide vanes. The difference being that a fusible mandrel asshown at 50 in FIG. 7 is placed in the center of the airfoil portion ofthe pre-form. The fusible mandrel should be a material that has amelting point which is above the curing temperature of the uncuredresin, but below the glass transition temperature of the cured resin. Itis possible to use high-temperature waxes of the type that have beenused in the past in “lost-wax” metal molding procedures. However, thepreferred fusible mandrel is made from a eutectic metal alloy that meltsover a relative narrow range (1 to 5° C.) and has a melting point thatis at least 1° C. above the temperature used to cure the resin andpreferably at least 5° C. above the temperature used to cure the resin.The upper end of the melting point range of the fusible material shouldbe at least 1° C. below the glass transition temperature of the curedresin and preferably at least 5° C. below the glass transitiontemperature of the cured resin.

Exemplary eutectic metal alloys that can be used to form the fusiblemandrel include eutectic mixtures of tin with zinc and/or bismuth thathave the above-mentioned melting properties. A preferred eutectic metalalloy for use as a fusible mandrel contains 91 weight percent tin and 9weight percent zinc. This tin/zinc alloy has a melting point of 199° C.Other eutectic metal alloys may be used provided that they meet themelting point requirements set forth above. Eutectic metal alloys can bemade from the base metals or can be purchased from commercial suppliers,such as Kapp Alloy & Wire, Inc. (Oil City, Pa.)

The pre-form is shaped around the fusible mandrel using the prepregtypes discussed previously. Although the pre-form can be composedentirely of randomly oriented segments of UD tape, it is preferred thatthe pre-form include UD tape prepreg in the airfoil portion and randomlyoriented segments of UD tape in the flanges as described above for thesolid outlet guide vane. The fusible mandrel should be located in thepre-form so that it is surrounded by equal thicknesses of prepregmaterial. It was found that the high pressures used during the moldingprocess can cause shifting of the mandrel if it is not surrounded byequal amounts of prepreg.

After the pre-form containing the mandrel has been cured, it is heatedfurther to a temperature that is sufficient to melt the fusiblematerial, so that it can drain out of the part through a hole that hasbeen drilled in the part. The part is cooled down after the meltedfusible material has been drained to form an outlet guide vane thatincludes a hollow chamber in the airfoil portion. The fusible mandrel 50is shaped to provide a single hollow chamber 28 as shown in FIG. 3. Ifdesired, multiple fusible mandrels may be located inside the pre-form toprovide multiple hollow chambers of the type shown at 32 and 34 in FIG.4. The formation of multiple chambers is preferred where it is desirableto provide additional structural strength to the airfoil portion in theform of a spar 36 in FIG. 4 or other type of rib structure. The multiplemandrels should also be surrounded by equal amounts of prepreg materialin the pre-form in order to prevent unwanted shifting of the mandrelsduring high pressure curing. In addition, a drain hole is required foreach mandrel to allow draining of the melted mandrel material from thechambers that are formed during molding of the part.

As previously mentioned, erosion protection can be added to the leadingedge of the outlet guide vane in the form of a thin sheet of protectivemetal 64 as shown in FIG. 6. The sheet of metal should be 0.001 to 0.010inch thick and should be made from a suitably strong and abrasionresistant metal or metal alloy. Stainless steel, titanium and othersimilar high-strength metal alloys are suitable. As a feature of theinvention, the protective metal sheet is placed in position on thepre-form prior to molding of the pre-form. The protective sheet isadhered to the pre-form using a thermoplastic adhesive that has asoftening temperature that is relatively low. Softening temperatures onthe order of 110° C. to 130° C. are preferred. It was discovered thatduring cool-down of the molded part, the metal protective sheet has atendency to debond from the composite airfoil portion. This problem isavoided by using an adhesive that does not solidify until the part hascooled down substantially from the cure temperature or the meltingtemperature of the mandrel when chambered parts are being made. It wasdiscovered that keeping the adhesive in a non-solid state during as muchof the cool-down as possible substantially reduces the tendency of theprotective metal to debond from the composite airfoil.

Exemplary adhesives that can be used to bond the leading edge protectivesheet to the outlet guide vane include polyolefin adhesives that have asoftening point of around 123° C. and polyamide adhesives that havesoftening point of around 116° C. Suitable polyolefin adhesives areavailable from BEMIS (Shirley, Mass.) under the trade name BEMIS 6343.Suitable polyamide adhesives are also available from BEMIS (Shirley,Mass.) under the trade name BEMIS 4220.

Examples of practice are as follows:

Example 1

A pre-form of the type shown at 40 in FIG. 8 was prepared in order toproduce an outlet guide vane of the type shown at 20 in FIGS. 2 and 3.The outlet guide vane had an airfoil portion that was 9 inches tall andranged in width from 2 inches to 4 inches. The thickness of the airfoilranged from 0.040 inch to 0.250 inch. The flanges were each about 5inches wide, 2 inches long and 0.230 inch thick. A mandrel of the typeshown at 50 in FIG. 7 was prepared from a eutectic metal alloy composedof 91 weight percent tin and 9 weight percent zinc (melting point—199°C.). The airfoil portion of the pre-form was prepared by covering themandrel with AS4/8552HexPly® unidirectional tape prepreg that had aresin content of 38 weight percent. The UD tape was laid so that itextended longitudinally between the flanges or platforms and overlappedinto the flanges by about 1.5 inch. The flange portions of the pre-formwere made from AS4/8552 HexMC® that had a resin content of 38 weightpercent. The overlapping sections of the UD tape were interleaved withthe layers of HexMC®. The pre-form was formed to closely resemble theoutlet guide vane with the pre-form being undersized by 0.25 inch in alldimensions, except the thickness of the airfoil portion. The mass of thecombined resin and fibers in the pre-form amounted to 103 weight percentof the desired outlet guide vane weight to account for weight loss dueto resin flashing during the curing process. It is preferred that themass of the pre-form be from 101 to 110 weight percent of the desiredoutlet guide vane weight.

The pre-form was initially placed in a thin gauge perforated sheet metaltool that closely resembled the curing mold in order to maintain theshape of the pre-form during staging of the resin. The tool-supportedpre-form was placed in an oven at 177° C. under ambient pressure for 9minutes to stage the resin and increase the minimum viscosity. Thestaged pre-form was cooled to room temperature and the sheet metaltooling removed. The staged pre-form was then placed in a compressionmold of the type shown at 52 and 54 in FIG. 5. The mold had beenpre-heated to a temperature of 196° C. Upon placement into the mold, thepre-form became molten and passed minimum viscosity within about 90seconds. After the initial 90 seconds, the mold was pressurized to 1500psi. The mold was held at this pressure and temperature for 20 minutes,after which the part was ejected. A hole was then drilled through thepart and into the chamber formed by the mandrel. The part was thenheated to 204° C. and kept there until the eutectic metal alloy wasmelted and drained from the chamber. The part was then cooled andsubjected to a variety of tests including compression testing. Thepredictive modal analysis of the outlet guide vane was found to matchvery closely the results of the modal testing. In addition, the lay-upand construction of the outlet guide vane could be adapted to avoidundesirable natural frequency response to the outlet guide vane based onspecific engine characteristics. The composite outlet guide vane wasfound to meet typical structural requirements of outlet guide vanes incompressive strength, resistance to buckling, fatigue resistance andnatural frequency in the free-state condition. If necessary, the flangesor platforms can be machined to remove flange material to meet designand/or dimensional tolerance requirements. Machining of the airfoilportion is not possible, since it would result in unacceptable damage tothe UD fibers that make up the airfoil.

Example 2

An outlet guide vane is prepared in the same manner as Example 1, exceptthat the entire pre-form is made from AS4/8552 HexMC® that has a resincontent of 38 weight percent. The pre-form is staged and cured in thesame manner as Example 1. The eutectic metal mandrel is melted andremoved as in Example 1 and the part is cooled to provide an allcomposite chambered outlet guide vane. This all composite part can bemachined in all areas to meet design and dimensional tolerances. Thepart is also expected to be equal to comparative metal outlet guidevanes in compressive strength, resistance to buckling, fatigueresistance and natural frequency in the free-state condition.

Example 3

A solid outlet guide vane of the type shown at 60 in FIG. 6 was made inthe same manner as Example 1, except that the mandrel was eliminated andAS4/8552HexPly® unidirectional tape prepreg was used instead to providea solid airfoil portion that was composed entirely of unidirectionalfibers. In addition, the leading edge of the pre-form was covered with aprotective sheet of 316 stainless steel that was 0.004 inch thick. Athin layer of BEMIS 4220 polyamide adhesive was located between theprotective sheet and the leading edge of the composite pre-form. Thepre-form was molded in the same manner as Example 1, except that theextra heating step to remove the fusible mandrel was not required. Theresulting solid outlet guide vane was also found to be better thancomparative metal outlet guide vanes in compressive strength andresistance to buckling. Although the hollow and solid outlet guide vaneshave the same geometry, they can be used to meet different structuralrequirement as needed depending on their location within the engine. Thesolid outlet guide vanes are stronger overall, but weigh more than thehollow outlet guide vanes. The weight of the airfoil portion of thesolid outlet guide vane is as much as twice that of the hollow outletguide vane. The protective stainless steel sheet remained securelybonded to the leading edge of the final outlet guide vane.

Comparative Example 1

A pre-form was prepared in the same manner as Example 1, except that themandrel was made from a eutectic metal alloy composed of 50 weightpercent tin and 50 weight percent bismuth. This eutectic metal alloy hada melting point of 138° C. The pre-form was molded in the same manner asExample 1, except that the mold temperature was 200° C. Since the moldcuring temperature was above the melting point of the eutectic metalalloy, the mandrel melted during the molding step. When the mold waspressurized to 1500 psi, liquid eutectic metal alloy was squeezedthrough the resin and fibers and out between the mold halves. As aresult, the internal chamber within the pre-form collapsed.

Comparative Example 2

A pre-form was prepared in the same manner as Example 1, except thatmore UD prepreg tape was placed on one side of the mandrel than on theother. The unbalanced pre-form was then molded in the same manner asExample 1. During the molding step, the high pressure in the mold causedthe extra prepreg on one side of the mandrel to exert excessive forceagainst the side of the mandrel that was lacking in prepreg. Thisunbalanced force in the mold caused the mandrel to be shifted anddistorted. Accordingly, as mentioned previously, it is preferred thatthe amount of prepreg material in the pre-form be balanced around themandrel to prevent shifting and distortion of the mandrel at the highpressures which are required to properly form high performance parts,such as outlet guide vanes.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention. Forexample, the invention may be used to make a variety of solid andchambered high performance parts where a combination of strength,machinability and light weight are required. In addition, the hollowchamber, which is formed when the fusible mandrel is melted and removedfrom the part, can be injection-filled with a foam or other fillermaterial that provides a desired property to the part, such as naturalfrequency dampening and acoustic dampening. Accordingly, the presentinvention is not limited by the above-described embodiments, but is onlylimited by the following claims.

What is claimed is:
 1. A moldable structure comprising: a moldable bodycomprising: a central portion comprising unidirectional fiberreinforcement and an uncured thermosetting resin that has a curingtemperature at which said uncured thermosetting resin is converted froma moldable resin to a solid resin having a glass transition temperaturethat is above said curing temperature, said central portion comprisingan internal surface that defines a chamber located within said centralportion; solid end portions located at each end of said central portion,said end portions comprising randomly oriented segments ofunidirectional fiber tape; and a solid mandrel located within saidchamber, said solid mandrel being in contact with said internal surfaceto provide shape thereto, said solid mandrel comprising a materialhaving a melting point at which said solid mandrel material melts toform a liquid material that can be drained out through a hole in saidinternal surface, said melting point being above said curing temperatureand below said glass transition temperature.
 2. A moldable structureaccording to claim 1 wherein the curing temperature of said uncuredthermosetting resin is between 170° C. and 225° C.
 3. A composite partcomprising a moldable structure according to claim 1 that has beenmolded to form said composite part and wherein said solid mandrel hasbeen melted to form said liquid and wherein said liquid material hasbeen drained from said chamber.
 4. A moldable structure according toclaim 2 wherein said solid mandrel material comprises a eutectic metalalloy.
 5. A moldable structure according to claim 1 wherein saidmoldable structure is in the shape of an outlet guide vane.
 6. Acomposite part according to claim 3 wherein said composite part is anoutlet guide vane.
 7. A moldable structure according to claim 4 whereinsaid eutectic metal alloy comprises tin and zinc or bismuth.
 8. Amoldable structure according to claim 7 wherein said eutectic metalallow is composed of 91 weight percent tin and 9 weight percent zinc.